1State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China
2State Key Joint Laboratory of Environment Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China
3College of Civil Aviation, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
4State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, 100029, China
5School of Physics, Peking University, Beijing 100871, China
Abstract. The main objective of this study is to investigate the formation and evolution mechanism of the regional haze in megacity Beijing by analyzing the process of a severe haze that occurred 20–27 September 2011. Mass concentration and size distribution of aerosol particles as well as aerosol optical properties were concurrently measured at the Beijing urban atmospheric environment monitoring station. Gaseous pollutants (SO2, NO-NO2-NOx, O3, CO) and meteorological parameters (wind speed, wind direction, and relative humidity (RH)) were simultaneously monitored. Meanwhile, aerosol spatial distribution and the height of planetary boundary layer (PBL) were retrieved from the signal of satellite and LIDAR (light detection and ranging). Results showed that high intensity of local pollutants from Beijing urban source is the fundamental cause that led to the regional haze. Meteorological factors such as higher RH, weak surface wind speed, and decreasing height of PBL played an important role on the deterioration of air quality. New particle formation was considered to be the most important factor contributing the formation of haze. In order to improve the atmospheric visibility and reduce the occurrence of the haze, the mass concentration of PM2.5 at dry condition should be less than 60 µg m−3 in Beijing according to the empirical relationship of visibility, PM2.5 mass concentration and RH. This case study may provide valuable information for the public to recognize the formation mechanism of the regional haze event over the megacity, which is also useful for the government to adopt scientific approach to forecast and eliminate the occurrence of regional haze in China.